The present invention relates generally to electrical motor/generators, and more particularly to machines including superconducting windings.
At least some known superconducting electric machines include a superconducting field coil installed on the rotor. The superconducting coil is maintained at a temperature approaching zero degrees Kelvin using a continuous supply of cryogenic fluid, such as, for example, but not limited to liquid helium (He2). If a high temperature superconductor (HTS) is used in fabricating the field coil, a cryogenic fluid such as nitrogen (N2) may be used to achieve superconducting temperatures. The cryogenic fluid is typically supplied to the superconducting field coil from a stationary cryocooler through a transfer coupling that is coupled to one end of the rotor. The transfer coupling channels the cryogenic fluid from a stationary portion to a rotating portion on the rotor. The cryogenic fluid is then routed through a cooling loop thermally coupled to the superconducting field coil and then back to the transfer coupling for return to the stationary cryocooler.
The superconducting field coil is subjected to thermal stresses, centrifugal stresses, and is provided with an electrical connection through the rotor to power the superconducting field coil. Accordingly, designing, fabricating and operating such a rotor may be difficult. For example, the superconducting coils, especially HTS coils, may be sensitive to mechanical strain. Specifically, because the coils are coupled to the rotor, the coils may be subjected to centrifugal forces that may cause strains and degrade the performance of the superconductor. In addition, because the coil is maintained at a cryogenic temperature, an elaborate support system may be needed to maintain the coil in position against the centrifugal forces while preserving the integrity of the thermal insulation between the coil and the parts of the rotor at ambient temperature.
It is desirable to overcome these shortcomings of the prior art. Further, it is desirable to provide a machine with improved performance characteristics, such as increased torque density.